1. Field of the Invention
The present invention relates generally to the field of analytical equipment and more particularly to an apparatus for ionizing analytes for introduction into mass spectroscopy and similar analytical devices.
2. Description of the Prior Art
Various analytical equipment separate analytes, and often their fractional components, according to a mass to charge ratio. Towards that end, the analyte is typically carried by a solvent into an ionizing chamber at the front end of the analytical unit. The analyte is ionized in the ionizing chamber and then the ions are accelerated by an electric field into the analytical unit for analysis. One type of ionizing chamber operates at or near atmospheric pressure and is therefore termed an Atmospheric Pressure Ionization (API) source.
FIG. 1 schematically illustrates a mass spectrometer 100 of the prior art. The mass spectrometer 100 includes an API source 110, a first ion guide 120, a second ion guide 130, and a mass analyzer 140. Ions produced in the API source 110 are directed by the ion guides 120 and 130 into the mass analyzer 140. The API source 110 includes a housing 150 that defines a chamber 160, a spray probe 170, and an exhaust port 180. The API source 110 also includes an ionizing mechanism (not shown).
In operation, a mixture of a solvent and an analyte for analysis is introduced as a fine droplet spray into the chamber 160 by the spray probe 170. Ideally, some of the analyte is ionized by the ionizing mechanism and drawn out of the chamber 160 by the first ion guide 120 while the remainder of the spray exits the chamber 160 through the exhaust port 180. In practice, however, various factors cause mixing between the spray and the atmosphere within the chamber 160. Accordingly, some of the analyte ends up circulating within the chamber 160 and deposits on the internal surfaces thereof.
The effect is two-fold. First, analyte circulating within the chamber 160 creates a memory effect whereby the intensity of the analyte that is read by the mass analyzer 140 will decay over a period of time after the introduction of the solvent/analyte mixture into the chamber 160 has ceased. Accordingly, if a second analyte is introduced too soon after the first, the first analyte will still appear in the reading. Secondly, analytes that deposit on interior surfaces of the chamber 160 slowly re-enter the chamber atmosphere and contribute to a background that reduces the signal to noise ratio.
U.S. Pat. No. 6,759,650 issued to Covey et al. attempts to address this problem through the use of an inner exhaust tube that extends from the exhaust port into the chamber. The leading edge of the inner exhaust tube is disposed close to the ion exit orifice that leads into the first ion guide. Disadvantageously, analytes that happen to collect on the leading edge of the inner exhaust tube can contaminate the atmosphere of the chamber. The concentrations of the contaminants are, of course, highest near the leading edge of the inner exhaust tube which is close to the ion exit orifice. Accordingly, this quickly leads to a reduction of the signal to noise ratio.
Therefore, what is needed is an API source with decreased recirculation of droplets.